1. Field of the Invention
The present invention relates to an articulated structure for a legged walking robot, and more particularly to an articulated structure for a legged walking robot, which includes hip joints having moving components whose inertial masses are reduced to effectively lower the energy required to actuate the moving components.
2. Prior Art
There are known legged walking robots having a plurality of legs, such as two-legged walking robots. A study of the motion of moving components of such a legged walking robot indicates that while the body of the legged walking robot moves at a substantially constant speed, the legs are repeatedly accelerated and decelerated to a great degree in each cycle of movement. Actuators associated with the respective articulations or joints of the legged walking robot are required to actuate the inertial masses of leg portions below those joints. Therefore, these joints need to actuate the leg portions with large torques at high speeds, which mean that a large amount of energy is consumed by the actuators to move the robot. However, the consumption by the walking robot of the large amount of energy is not preferable because it reduces the usefulness of the robot.
The legs of a legged walking robot include joints for making motions in a forward direction (hereinafter referred to as a "pitch direction"), a lateral direction (hereinafter referred to as a "roll direction") or a rotational direction, etc. The actuators for actuating the leg portions in the pitch direction are most required to produce large torques at high speeds. This is because the leg portions which are movable in the pitch direction move in a wide range and hence at large accelerations and decelerations, so that they are required to move with large torques at high speeds or high frequencies. As a result, the joints for moving the leg portions in the pitch direction and actuator motors associated with these joints are large and heavy.
Generally, a legged walking robot has a vertical series of joints. Use of actuators which produce large torques at high speeds in combination with the vertical series of joints increases the inertial masses of leg portions that have to be actuated by the actuators in higher positions. Therefore, the actuators in higher positions must be of greater capabilities, with the result that the total weight of the legged walking robot is increased. In connection with the above problem, there has been proposed a legged walking robot which includes intersecting pitch and roll axes in each of its hip joints, as disclosed in Japanese Laid-Open Patent Publications Nos. 62(1987)-97005 and 62(1987)-97006. Each hip joint comprises a hydraulic pump and a hydraulic actuator. However, while the hydraulic actuator is small and powerful, it causes a large energy conversion loss, and the hydraulic pump has to be operated by an electric motor of large capability. Such an actuator assembly is not preferable for the legged walking robots for which only a limited amount of energy is available. At any rate, the proposed legged walking robot does have an articulated structure designed to reduce the inertial masses.
The hip joints of a legged walking robot are complex because they need to have many degrees of freedom. The mechanical components of the legs of the legged walking robot tend to interfere with each other when the joints are actuated. To avoid such a physical interference between the mechanical components, it is necessary to design the robot such that the mechanical components be spaced from each other, e.g., the legs be spaced from each other by a large distance. If the legs are largely spaced from each other, then when one of the legs is in a transfer phase, the weights of the leg in the transfer phase and the body of the robot are applied to the other leg which is in a support phase. With the hip joint of the leg in the support phase, the joint for driving the leg in the roll direction is subjected to the moment caused by the applied weights. Since the imposed moment increases in proportion to the distance by which the legs are spaced apart, the electric motors for actuating the joints on the legs are required to produce large output powers, and the inertial masses to be driven by the motors are large, resulting in an increased amount of energy consumed by the robot. The spaced-apart legs also increase an inertial moment about the vertical axis of the robot body. As a consequence, the robot cannot move lightly.
Legged walking robots have foot and knee joints as well as hip joints. These foot and knee joints are also actuated in the pitch and roll directions. It is desirable that the path along which a leg in the transfer phase is to move be easily calculated in the gait control of the robot. The three joints, i.e., the foot, knee, and hip joints associated with each leg, in the conventional legged walking robots are arranged parallel to each other, making it possible to calculate the path of movement in an orthogonal coordinate system. Since a yaw axis for changing the direction of movement of the robot is disposed on each of the thighs of the robot, however, when the robot is turned, the parallelism of the pitch axes is disturbed by an interference with the yaw axes, with the result that the necessary calculations are rendered highly complex.
The actuators in a legged walking robot are advantageously positioned upwardly of the associated joints in order to reduce the inertial moments applied lower leg portions. With such a construction, the robot has a high center of gravity. Therefore, when a gravitational pull is exerted to the robot, tending to overturn the upper body of the robot at the time the robot is about to stand up, the time required for the robot to fall as an inverted pendulum is long enough for the robot to be controlled to achieve its normal posture. For this reason, it has been proposed to position electric motors upwardly of associated joints and to drive the articulations with belts powered by the motors at suitable power transmitting ratios, as disclosed in Control of Dynamic Two-Legged Walking Robots, written by Furusho, Bulletin No. 3, Vol. 1, of Japan Robotics Society.
According to the above known arrangement, since torque of the motors is transmitted by the belts whose speed reduction ratio is usually limited to 3 to 4, the electric motors which are designed to rotate at high speed have to be energized in a low speed range. Inasmuch as the desired torque cannot be produced from the electric motors because of the available speed reduction ratio, the capacity of the electric motors has to be increased. Consequently, the proposed robot structure is not effective enough to reduce the inertial moments. The torque required by the joints is determined necessarily in a quantity required to support the weight of the robot when the robot is about to stand up. The belts which transmit the torque to the joints are thus subjected to strong forces, and hence have to be wide and large enough to withstand the applied forces. With the wide and large belts incorporated in the hit joints, the legs must be designed such that the robot walks bandy-legged in order to avoid the physical interference between the legs.